once again,Alfons is spared, given a second chance, why? How do you know?

Because so many men had died, both very young and older men were called to the military as replacements. Alphonse had made the mistake of ignoring military orders, and he was punished for his mistake. The demand for men allowed him a second chance to become a pilot; the military couldn't afford to lose him. Thus, he got a second chance.